Fix accounting in interference::Graph::combine
Fix accounting in interference::Graph::combine

Summary:
- only add edges after making weight-affecting accounting changes
- combine spill costs

Reviewed By: beicy

Differential Revision: D38606399

fbshipit-source-id: dda80ae55c48c3b9015b633aaa8a4acc8dfb16f7

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex